A Cr-containing austenitic alloy tube has been used as various members because of being excellent in mechanical properties. In particular, since the members for a nuclear reactor is exposed to high-temperature water, a Cr-containing austenitic alloy tube excellent in corrosion resistance has been used as a member for a nuclear reactor. For example, as a member of a steam generator for a pressurized water reactor (PWR), a 60% Ni-30% Cr-10% Fe alloy or the like has been used.
These members are used in an environment of high-temperature water of about 300° C. which is a nuclear reactor water environment, for several years to several tens years. For the Cr-containing austenitic alloy tube used as a steam generator tubing for nuclear power plant, although Ni is contained much, and therefore the corrosion resistance is excellent and the corrosion rate is low, a minute amount of Ni is eluted from a base metal by the long-term use.
In a process in which reactor water circulates, the eluted Ni is carried to a reactor core and receives neutron irradiation in the vicinity of fuel. When receiving neutron irradiation. Ni is converted into radiocobalt by nuclear reaction. This radiocobalt continues to emit radioactive rays for a long period of time because the half-life thereof is very long. Therefore, if the elution amount of Ni is large, regular inspection cannot be started until the emitted radiation dose decreases to a proper level, so that the period of regular inspection extends, which results in an economic loss.
To reduce the exposure dose is a very important issue in using a light water reactor for a long period of time. So far, therefore, measures have been taken to prevent Ni in the Cr-containing austenitic alloy tube from elution by improving the corrosion resistance on the material side and by controlling the quality of reactor water.
Patent Document 1 discloses a method in which Ni-based alloy heat-transfer tube is annealed in the temperature range of 400 to 750° C. in an atmosphere having a degree of vacuum of 10−2 to 10−4 Torr to form an oxide film consisting mainly of chromium oxides, whereby the general corrosion resistance is improved.
Patent Document 2 discloses a method far producing a member for nuclear power plant, in which after the solution treatment of a Ni-based precipitation strengthened alloy, beating treatment is performed combinedly with at least part of age hardening treatment and oxide film forming treatment in an oxidizing atmosphere of 10−3 Torr to atmospheric pressure.
Patent Document 3 discloses a method for producing a Ni-based alloy product, in which a Ni-based alloy product is heat-treated in an atmosphere of hydrogen or a mixed atmosphere of hydrogen and argon, the atmosphere having a dew point of −60° C. to +20° C.
Patent Document 4 discloses a method for forming a chromium-rich layer by exposing an alloy workpiece containing Ni and Cr to a gas mixture of water vapor and at least one kind of nonoxidizing gases.
Patent Document 5 discloses a method of heat treatment in which an oxide film of two-layer structure for restraining the elution of Ni is produced reliably and efficiently on the inner surface of a Ni-based alloy tube in a high-temperature water environment. In this method, at least two gas feeding devices are provided on the exit side of a continuous heat treatment furnace, or one gas feeding device is provided on each of the exit side and the entrance side. The tube is charged into the furnace and held at a temperature of 650 to 1200° C. for 1 to 1200 minutes while feeding an atmospheric gas consisting of hydrogen or a mixed gas of hydrogen and argon, the atmospheric gas having a dew point in the range of −60° C. to +20° C., from the front end side in the travel direction into a work tube before being charged into the heat treatment furnace by using one device of the gas feeding devices and a gas introducing tube penetrating the interior of the furnace. In the above process, after the front end of tube has arrived at the exit side of furnace, an operation of changing over the feed of atmospheric gas into the interior of tube to the feed from the other gas feeding device is repeated.
Patent Document 6 discloses a method for producing a Ni-based alloy, in which a Ni-based alloy is treated in a heating treatment atmosphere consisting of carbon dioxide gas or an atmosphere consisting of at least one of 0.0001 vol % or more of carbon dioxide gas, 99.9999 vol % or less of hydrogen gas, and 99.9999 vol % or less of rare gas, whereby an oxide film consisting of chromium oxides is formed on the surface of the Ni-based alloy.
Patent Documents 7 and 8 disclose a method for producing a Cr-containing nickel-based alloy tube, in which the Cr-containing nickel-based alloy tube is treated in an atmosphere consisting of nonoxidizing gas containing carbon dioxide, whereby a chromium oxide film having an intended thickness is formed on the inner surface of tube.